1. Field of the Invention
The present invention relates to a crystallization apparatus and a crystallization method which produce a crystallized semiconductor film by irradiating an uncrystallized semiconductor film such as a polycrystalline semiconductor film or an amorphous semiconductor film with radiant energy such as a laser beam. More particularly, the present invention relates to a crystallization apparatus and a crystallization method which produce a crystallized semiconductor film by irradiating an uncrystallized semiconductor film with a laser beam phase-modulated by using a phase shift mask.
2. Description of the Related Art
Materials of a thin film transistor (TFT) for use in a switching element or the like which controls a voltage to be applied to pixels of, e.g., a liquid crystal display (LCD) are conventionally roughly classified into amorphous silicon, polycrystalline silicon and monocrystalline silicon.
Polycrystalline silicon has a higher mobility of electrons or electron holes than the amorphous silicon. Therefore, when a transistor is formed by using polycrystalline silicon, since the switching speed is higher than that in the case of using amorphous silicon, there is an advantage that the response of a display becomes fast and the design margin of any other component can be reduced. Further, when peripheral circuits such as a driver circuit formed at a part other than a display main body or a DAC which converts a digital signal into an analog signal are formed within a display area, using the amorphous silicon enables high-speed operation of these peripheral circuits.
Although polycrystalline silicon comprises a set of crystallized grains, it has a lower mobility of electrons or electron holes than monocrystalline silicon. Furthermore, in a small transistor formed by using the amorphous silicon, there is a problem of irregularities in crystal grain boundary number at a channel portion. Thus, in recent years, in order to improve the mobility of electrons and electron holes and reduce irregularities in crystal grain boundary number at a channel portion (activation portion) of each TFT, many crystallization methods which produce crystal grains with a larger particle size have been proposed.
As this type of crystallization method, there has been conventionally known phase control ELA (Excimer Laser Annealing), which produces a crystallized semiconductor film by irradiating a polycrystalline semiconductor film or an amorphous semiconductor film with an excimer laser beam through a phase shift mask. The details of phase control ELA are disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278–487, 2000” or Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 ([0030] to [0034], FIGS. 8 to 11).
In phase control ELA, the phase shift mask is used to generate a light intensity distribution of an inverse peak pattern (light intensity distribution where the light intensity is rapidly increased with distance from the position where the light intensity is minimum), and an uncrystallized semiconductor film such as a polycrystalline semiconductor film or an amorphous semiconductor film is irradiated with a light beam cyclically having this light intensity distribution of the inverse peak pattern. As a result, a fusion area is generated in the irradiated uncrystallized semiconductor film in accordance with the light intensity distribution, a crystal nucleus is formed at a part which is not fused or a part which first solidifies in accordance with the position where the light intensity is minimum, and the crystal grows in a lateral direction (lateral growth) from the crystal nuclei toward the circumference, thereby generating crystal grains (monocrystalline) with a large particle size.
As described above, in the prior art, since the semiconductor film is irradiated with a light beam that has the light intensity distribution of the inverse peak pattern and the crystal nuclei is formed at a part corresponding to a position where the light intensity is minimum in the light intensity distribution, the crystal nuclei formation position can be controlled.
However, the phase shift mask cannot control the light intensity distribution at an intermediate portion between two adjacent inverse peak pattern portions.
Actually, in the prior art, the light intensity distribution at the intermediate part generally involves irregular surges (undulance distribution such that an increase and a decrease in the light intensity are repeated). In this case, in a crystallization process, there is a disadvantage that the lateral growth, which has started from the crystal nucleus, stops at the intermediate portion where the light intensity decreases, so that growth of a large crystal is interrupted. Furthermore, even if a substantially uniform light intensity distribution is obtained at the intermediate portion, there is a drawback that the lateral growth is stopped at an arbitrary position in this uniform light intensity distribution and growth of a large crystal is interrupted.